You have obviously forgotten that the Constitution is a living document. That means they get to take an oath to uphold their Constitution, not your musty, moldy old Constitution.
That living document thing has always seemed dumb to me. Yes we have amendments, and the courts interpret it too. But that's not what they mean by living document. To swear an oath to it, but only the parts YOU believe in, is just saying "I promise to believe what I believe today". It's not an oath at all, you might as well take an oath that you like ice cream.